Human immunodeficiency virus (HIV) has been identified as the etiological agent responsible for acquired immune deficiency syndrome (AIDS), a fatal disease characterized by destruction of the immune system and the inability to fight off life threatening opportunistic infections. Recent statistics (UNAIDS: Report on the Global HIV/AIDS Epidemic, December 1998), indicate that as many as 33 million people worldwide are infected with the virus. In addition to the large number of individuals already infected, the virus continues to spread. Estimates from 1998 point to close to 6 million new infections in that year alone. In the same year there were approximately 2.5 million deaths associated with HIV and AIDS.
There are currently a number of antiviral drugs available to combat the infection. These drugs can be divided into four classes based on the viral protein they target and their mode of action. In particular, saquinavir, indinavir, ritonavir, nelfinavir atazanavir darunavir, amprenavir, fosamprenavir, lopinavir and tipranavir are competitive inhibitors of the aspartyl protease expressed by HIV. Zidovudine, didanosine, stavudine, lamivudine, zalcitabine, emtricitibine, tenofovir and abacavir are nucleoside reverse transcriptase inhibitors that behave as substrate mimics to halt viral cDNA synthesis. The non-nucleoside reverse transcriptase inhibitors, nevirapine, delavirdine, efavirenz and etravirine inhibit the synthesis of viral cDNA via a non-competitive (or uncompetitive) mechanism. Enfuvirtide and maraviroc inhibit the entry of the virus into the host cell. Used alone these drugs are effective in reducing viral replication. The effect is only temporary as the virus readily develops resistance to all known agents. However, combination therapy has proven very effective at both reducing virus and suppressing the emergence of resistance in a number of patients. In the US, where combination therapy is widely available, the number of HIV-related deaths has declined (Palella, F. J.; Delany, K. M.; Moorman, A. C.; Loveless, M. O.; Furher, J.; Satten, G. A.; Aschman, D. J.; Holmberg, S. D. N. Engl. J. Med. 1998, 338, 853-860).
Unfortunately, not all patients are responsive and a large number fail this therapy. In fact, approximately 30-50% of patients ultimately fail combination therapy. Treatment failure in most cases is caused by the emergence of viral resistance. Viral resistance in turn is caused by the rapid turnover of HIV-1 during the course of infection combined with a high viral mutation rate. Under these circumstances incomplete viral suppression caused by insufficient drug potency, poor compliance to the complicated drug regiment as well as intrinsic pharmacological barriers to exposure provides fertile ground for resistance to emerge. More disturbing are recent findings which suggest that low-level replication continues even when viral plasma levels have dropped below detectable levels (<50 copies/ml) (Carpenter, C. C.; Cooper, D. A.; Fischl, M. A.; Gatell, J. M.; Gazzard, B. G.; Hammer, S. M.; Hirsch, M. S.; Jacobsen, D. M.; Katzenstein, D. A.; Montaner, J. S.; Richman, D. D.; Saag, M. S.; Schechter, M.; Schooley, R. T.; Thompson, M. A.; Vella, S.; Yeni, P. G.; Volberding, P. A. JAMA 2000, 283, 381-390). Clearly, there is a need for new antiviral agents, preferably targeting other viral enzymes to reduce the rate of resistance and suppress viral replication even further.
HIV expresses three enzymes, reverse transcriptase, an aspartyl protease, and integrase. All three are targets for treating AIDS and HIV infection. HIV integrase is a component of the pre-integration complex of the virus that is assembled in the cell shortly after infection (Chiu, T. K.; Davies, D. R. Curr. Top. Med. Chem. 2004, 4, 965-977). This enzyme catalyzes the integration of proviral DNA into the host genome and is absolutely required for viral infectivity. Early experiments showed that mutating the active site of integrase within a proviral clone produces virus unable to replicate due to its inability to insert into the host chromosome (Englund, G.; Theodore, T. S.; Freed, E. O.; Engleman, A.; Martin, M. A. J. Virol. 1995, 69, 3216-3219). Selective HIV integrase inhibitors have been shown to possess effective anti-HIV activity in cell culture (Hazuda, D. J.; Felock, P.; Witmer, M.; Wolfe, A; Stillmock, K.; Grobler, J. A.; Espeseth, A.; Gabryelski, L.; Schleif, W.; Blau, C.; Miller, M. D. Science, 2000, 287, 646-650), and it is clear that this class of inhibitors is very effective as part of a combination regimen containing HIV inhibitors of different classes. An HIV integrase inhibitor, raltegravir (MK-0518, Isentress®), has been approved for use in treatment experienced patients based upon 48 week trial results (Cooper, D. A.; Gatell, J.; Rockstroh, J.; Katlama, C.; Yeni, P.; Lazzarin, A.; Xu, X.; Isaacs, R.; Teppler, H.; Nguyen, B. Y. 15th Conference on Retroviruses and Opportunistic Infections, Boston, Mass., Feb. 3-6, 2008 Abst. 105LB: Evering, T. H.; Markowitz, M. Drugs Today, 2007, 43, 865-877). In addition, a second integrase inhibitor, elvitegravir (GS-9137), completed a successful Phase II trial in combination with ritonavir boosting in naive and treatment experienced patients (Zolopa, A. 14th Conference on Retroviruses and Opportunistic Infections, Los Angeles, Calif. Feb. 25-28, 2007 Abst. 143LB). Thus, HIV-1 integrase is a promising target for novel anti-HIV-1 therapeutics.